Support creating FCS files with data type `I`

[whitews]

The create_fcs function only supports creating FCS files with data type F. Though less common these days, there are still FCS files with data type I around. Plus, FCS 3.2 does allow mixed types and a major use case for doing so is to label events with integer values, so this should be implemented in such a way that allows future functionality to apply datatypes on a channel-by-channel basis.